So far, the scientists have tested four cities and found microplastics — pieces of plastic roughly the size of a sesame seed or smaller — in all of their samples. The researchers expect that the pattern will continue and every city they test will show microplastics contaminating the air, according to The Guardian. Scientists do not yet know what health effects, if any, are associated with breathing in microplastics, though they do know that air pollution can damage nearly every organ and affect every cell in the human body, according to a comprehensive global review published earlier this year.

The scientists were surprised about just how much plastic was found in the air.

"We found a high abundance of microplastics, much higher than what has previously been reported," said Stephanie Wright, from Kings College London, who led the research, to The Guardian. "But any city around the world is going to be somewhat similar."

"I find it of concern – that is why I am working on it," she added. "The biggest concern is we don't really know much at all. I want to find out if it is safe or not."

The concentration of microplastics in London was alarming. The researchers collected microplastics falling onto the roof of a nine-story building in central London. They found a range of 575 to 1,008 pieces of plastic per square meter per day, according to the research published in the journal Environment International, as The Guardian reported. By contrast, when researchers found microplastics in the Pyrenees in southern France, they found roughly 11,400 pieces per square meter in a month, which is about one-third of what was collected in London.

"We kind of expected to find plastics there, but we certainly were not prepared for the numbers we found," said Deonie Allen, one of the lead researchers in the Pyrenees study, to The New York Times. "It was astounding: 11,400 pieces of microplastic per square meter per month, on average."

Another of the researchers in that study, Steve Allen, at the EcoLab research institute near Toulouse, France, called the findings scary. He also read Dr. Wright's study about urban microplastic pollution.

"These studies showing just how much plastic is in the air are a wake-up call," said Allen to The Guardian. "The [London research] is a very well done study showing incredibly high numbers of airborne microplastics."

What Dr. Wright and her team found in London comprised 15 different types of plastics. Most were acrylic fibers, likely from clothes. Only 8 percent of the microplastics were particles like polystyrene and polyethylene, which are ubiquitous in food packaging, as The Guardian reported.

Furthermore, the plastics they found were between 0.02mm and 0.5mm. That's large enough to enter the airways or to get trapped and swallowed in saliva. However, smaller particles that are particularly damaging to the lungs and can enter the bloodstream were too small to be measured by current technology, according to The Guardian.

"Currently we have very little knowledge on what effect this airborne pollution will have on humans," said Allen to The Guardian. "But with what we do know it is pretty scary to think we are breathing it in. We need urgent research."